We have a comment saying to replace the simple binary_semaphore type
with std::binary_semaphore, which has been done. However, that isn't
defined on all targets. So keep the simple one here that just implements
the parts of the API needed by <stop_token>, and remove the comment
suggesting it should be replaced.

libstdc++-v3/ChangeLog:

	* include/std/stop_token: Remove TODO comment.

Tested powerpc64le-linux. Committed to trunk.